1. Field of the Invention
This invention relates generally to connections between components, and more particularly to connection arrays where each connection in the array includes a bump inserted into a well containing bonding material.
2. Description of the Related Art
The number of input/output (I/O) connections required by integrated circuit (IC) chips is increasing, to several hundred for recent microprocessor chips. As verification of complex designs becomes an increasing portion of the total design activity, it is desirable to increase the I/O count further, to provide access to more internal nodes for testing. Flip chip assembly methods have helped to provide more I/O connections because they provide an area array of connections across the entire face of an IC chip, rather than just at the perimeter as with wire bonding. However, even with flip chip assembly it continues to be desirable to reduce the pad pitch, the distance between bonding pad centers, in order to achieve more I/O connections per unit area of IC chip.
A recent advance in flip chip assembly capability has been the introduction of stud bumping machines that can provide gold stud bumps on IC chips with pad pitches of less than 100 microns. However, to take advantage of this capability, the system board or package that receives the bumped devices must have fine traces in order to route all of the signals in the space available with not too many layers, plus it must support bonding pad pitches less than 100 microns. The most recent packaging technology to be commercially introduced is called land grid array, LGA. It builds up the wiring layers by plating a base layer of copper that has been patterned with photo resist. The external terminal pitch claimed for this packaging method is “less than 0.5 mm”. This is not sufficient for direct bonding of IC chips; in fact none of the available printed circuit board technologies can support direct mounting of bumped devices at a pitch of 100 microns or less. The current invention is capable of pad pitches of less than 100 microns without using any redistribution wiring layers; it also includes a viable method for reworking defective IC chips at this bonding density.
Indium-based solders have been developed that form reliable bonds with gold structures such as stud bumps. An example is Indalloy 290 available from Indium Corporation of America, Utica, N.Y. This alloy contains 97% indium and 3% silver and melts at 143° C. Since the solubility of gold in this solder is very low, brittle inter-metallic compounds are not a significant problem, and the integrity of a gold stud bump can be maintained through multiple rework cycles.
For many years the minimum trace width available from printed circuit board vendors has been around 100 microns using glass epoxy laminates. Recently, Unitive, Inc. of Research Triangle Park, N.C., USA, has produced interconnection circuits using a spin-on dielectric called BCB (benzocyclobutene) having a copper trace width of 12 microns and a space between traces of 13 microns. The current invention preferably uses aluminum conductors and BCB dielectric and is capable of achieving trace widths of 5 microns or less, together with a trace pitch of 10 microns or less. These fine aluminum traces are well suited for low power and low frequency applications. Higher current applications may use thicker and wider traces, and may substitute copper for aluminum. High frequency applications can employ the methods described herein if additional effort is applied to form traces with controlled impedance; for example, if differential pairs and ground reference planes are employed the frequency range may be extended to the order of 1 GHz, depending on trace lengths and other specifics of the application.
One way to achieve fine line interconnection circuits is to employ a semiconductor fabrication facility and to build the interconnection circuit on a silicon wafer; hence the term, wafer level packaging, WLP. The precision of the associated photolithographic methods, the clean room environment with low particulate count, and the advanced substrate handling equipment of such a facility can all contribute to high-density interconnection circuits. However, the application of IC chip manufacturing facilities to this problem is more than what is required. An intermediate alternative is to apply the manufacturing resources of a glass panel fabrication facility, where the minimum feature sizes are 10 to 20 times larger than for IC chips (but still adequate for the most advanced assembly processes), and the manufacturing cost per unit area is less than 5% of the cost per unit area of IC chips. In addition, the glass panel fabrication facility can produce system boards of any size up to around two meters on a side for the latest panel fabrication facilities, whereas the largest wafers produced have a diameter of 300 mm.
In order to avoid the rigidity and weight of the glass substrate, and to provide better thermal access to the heat producing components for cooling them, it is usually preferable to discard the glass carrier after most of the processing is done.
Typically, the fine trace capability of WLP has been used to create redistribution circuits that map from the fine pitch available with flip chip bonding to the coarser pitch of a printed circuit board. The current invention eliminates the redistribution circuits because the printed circuits produced (termed interconnection circuits) include fine features that easily accommodate the fine pitch of the flip chip bonding.
Power supply voltages and signal voltage swings are reducing with each new generation of IC chip technology. To achieve the necessary noise margins during testing it is generally necessary to connect test points to test circuits using short leads. This is typically achieved by employing a test head that is located close to the test points. The test head provides a set of pin electronics for each signal tested; the pin electronics typically include high-speed sampling circuits and comparator circuits, along with matrix switches and relays to map test points to test pins. Providing test chips on the motherboard (system board) is another way to achieve high speed functional testing. The test chip or chips can be placed close to system buses for sampling signal activity at high speed, and this test method is recommended for electronic modules of the current invention. Said test chips incorporate high speed sampling circuits and comparators; they work in concert with a test support computer that is cabled to the system and communicates with the test chips and the system at relatively low data rates. More accurate and complete testing of components is provided when they are tested in their real system environment rather than being tested as individual components using test vectors that represent a simulation of the system environment. The system environment is preferably created with the actual system running a test version of the system's application code, programmed in the language of the application rather than a special test language. This way, the system architects can also be the test architects. This can lead to improvements in test development time, test effectiveness and cost, compared with the conventional approach that includes a test program in a specialized test language, simulated test vectors, and a general-purpose tester. This test method provides focus on the system level requirements, as opposed to component level requirements. If the system level requirements are satisfactorily met, then the minutiae of component level characteristics become irrelevant. Alternatively stated, only the functions relevant to proper system function are tested; this is a much more manageable set of requirements than the total set of functions that all the assembled components are capable of performing.
If necessary, multiple IC chips may be employed to test the entire range of digital, analog, and RF functions of a particular product. Adding these chips to the system using the current invention is not as expensive as in the past because the test chips will be manufactured in volume and will have low unit costs, and the packaging and assembly cost will be minimal, as will be further discussed. In summary, a mini-tester may be included with every module produced, but the cost of this tester may be well justified by the system level assembly and performance benefits, and the reduction in system development time.
Module verification can be performed at an elevated temperature by heating the glass carrier underneath the module (circuit assembly or assembled system board). By providing a pre-determined test temperature to the entire circuit assembly, a speed grade can be associated with the module, as has been done in the past at the component level. Greater emphasis can be placed on environmental stress testing at the module level. Accelerated life testing can also be performed early in the life cycle of a product, and lessons learned about particular components can be incorporated into the module level test.
R. K. Traeger, “Hermeticity of Polymeric Lid Sealants”, Proc. 25th Electronics Components Conf., 1976, p. 361, has documented the water permeabilities of silicones, epoxies, fluorocarbons, glasses and metals. Traeger's data shows that, in terms of providing a barrier to water, a layer of metal that is 1 micron thick is approximately equivalent to a layer of glass that is 1 mm thick, and also equivalent to a layer of epoxy that is 100 cm thick. Hermetic packaging techniques and electromagnetic shielding techniques can be applied at the module level to improve both performance and manufacturing cost. The current invention describes a method for fabricating a metal envelope that encloses almost the entire module, substantially attenuating the interference from individual components and the wiring between them. Cost can be reduced because hermeticity and shielding are provided with a simple process applied once to the entire system, rather than being addressed individually at each of the components.
Such a complete module or system fabrication process can be achieved if a panel fabrication facility is applied to the complete set of module manufacturing steps, including high-density cables and connectors and back-end processing for hermeticity and shielding. Sub-processes for the following structural elements are included: multi-layer interconnection circuits; a special assembly layer that may be required for direct attachment of IC chips (wells filled with solder at each I/O pad); module access ports (arrays of test points and system interconnects having wells filled with solder at each I/O pad); module access cables or test fixtures for connecting between the module access ports and external systems; and the back end module layers that provide both hermeticity and electromagnetic shielding.
Usually structures for direct chip attach require an epoxy under-layer between direct mounted IC chips (flip chips) and the package or circuit board. The purpose of the under-layer is to provide mechanical strength to withstand repeated thermal cycling without developing cracks in the area of the flip chip bonds. The thermal stress typically arises because of differences in coefficients of thermal expansion (CTEs) between the IC chip material and the board material. Gelatinized solvents have been used to dissolve the epoxy during rework of defective chips; they typically leave a residue that must be cleaned off. The process of cleaning off the epoxy and the residue has often resulted in damage to the fine pitch bonding leads, to the point where they cannot be reliably re-bonded. This under-layer is unnecessary with the current invention because mechanical strength is provided at each bond by the physical structure of a stud bump mated with a well filled with solder. Also the final interconnection circuit is flexible so that thermally induced stresses are substantially eliminated in the region of the flip chip bonds. Without the thermally induced stress, no cracking will occur. Thermal stresses are still present during assembly (because the carrier/interconnection circuit is rigid at this point), but are avoided during operation in the field (when the carrier is removed and the interconnection circuit is flexible). The number and extent of thermal cycles endured during assembly are more predictable and controllable than thermal cycles arising from operation in the field. Stress testing in the laboratory can be used to quantify the acceptable temperature limits, and assure crack-free circuit assemblies. However, using the proposed flip chip assembly method, it may be necessary to limit the maximum size of IC chips assembled, to limit the maximum strain induced by thermal mismatch during assembly.
Replacement of defective chips (rework) is much easier if there is no epoxy under layer to be removed. Also, in the preferred embodiment there are no delicate traces to be damaged during rework because each I/O pad is provided with a well filled with solder, as will be further described. Attaching IC chips onto flexible substrates is referred to in the art as “compliant packaging”.